Allow setting rpath to libgnat-12.so, because it isn't added to ldconfig directories.

--- Makefile.in.orig	2025-03-01 16:49:42 UTC
+++ Makefile.in
@@ -485,7 +485,7 @@ lib/$(libghdl_name): $(GRT_SRC_DEPS) $(LIBGHDL_GRT_OBJ
 lib/$(libghdl_name): $(GRT_SRC_DEPS) $(LIBGHDL_GRT_OBJS) version.ads force
 #	Use -g for gnatlink so that the binder file is not removed.  We need
 #	it for libghdl.a
-	$(GNATMAKE) -I- -aI. -D pic -z libghdl -o $@ -gnat12 $(GNATFLAGS) $(PIC_FLAGS) $(LIBGHDL_INCFLAGS) -bargs -shared -Llibghdl_ -largs -R -g -shared $(SHLIB_FLAGS) $(filter-out -static,$(LDFLAGS)) $(LIBGHDL_GRT_OBJS)
+	$(GNATMAKE) -I- -aI. -D pic -z libghdl -o $@ -gnat12 $(GNATFLAGS) $(PIC_FLAGS) $(LIBGHDL_INCFLAGS) -bargs -shared -Llibghdl_ -largs -g -shared $(SHLIB_FLAGS) $(filter-out -static,$(LDFLAGS)) $(LIBGHDL_GRT_OBJS)
 #       On windows, gnatmake (via Osint.Executable_Name) always appends .exe
 #       Adjust.  (Other solution: use gnatmake for compilation and binding,
 #       then use gnatlink directly for linking).